1. Field of the Invention
An aspect of the present invention relates to an optical recording and/or reproducing apparatus, and in particular to an optical pick-up actuator having a servo control system with increased stability.
2. Description of the Related Art
An optical pick-up typically used to record and/or reproduce information onto and/or from an optical medium such as a CD (Compact Disc) or a DVD (Digital Versatile Disc) includes an optical system having an objective lens to focus light from a light source onto an optical medium to record and/or reproduce information onto and/or from the optical medium, and an optical pick-up actuator to adjust a distance between the objective lens and the optical medium, and a position and a tilt of an optical axis of the objective lens.
The optical pick-up actuator performs servo control including focusing control to maintain a constant distance between the optical medium and the objective lens, tracking control to control a position of the optical axis of the objective lens in a radial direction of the optical medium, and tilting control to make the optical axis of the objective lens perpendicular to the optical medium. The focusing control is performed by moving a lens holder on which the objective lens is mounted toward and away from the optical medium to control the distance between the objective lens and the optical medium, and the tracking control is performed by moving the lens holder in the radial direction of the optical medium. The tilting control is performed by tilting the lens holder in the radial direction of the optical medium about a tilting axis parallel to a tangential direction of the optical medium, known as “radial tilt,” and/or tilting the lens holder in the tangential direction of the optical medium about a tilting axis parallel to the radial direction of the optical medium, known as “tangential tilt.”
For the purpose of performing the servo control described above, the optical pick-up actuator has plural magnet members and plural coils. If electric currents are supplied to the coils, the position and tilt of the lens holder are controlled by electromagnetic forces generated by interactions between the magnet members and the coils having the electric currents supplied thereto. Two or more pairs of magnet members may be used to generate such electromagnetic forces. In order to miniaturize the optical pick-up actuator, grooves may be formed in the lens holder and at least some of the magnet members may extend into the grooves. In such a case, it is necessary to maintain a certain gap between the walls of the grooves and the magnet members used to perform the tilting control and/or the tracking control. This gap serves as a tracking margin for performing the tracking control, and also serves as a tilting margin for performing the tilting control. The tracking margin is a space in which the lens holder is free to move in the radial direction of the optical medium without contacting any part of the optical pick-up actuator while the tracking control is being performed, and the tilting margin is a space in which the lens holder is free to tilt about the tilting axis without contacting any part of the optical pick-up actuator while the tilting control is being performed.
When the tilting control and the tracking control are performed, however, the gap may be reduced as the lens holder is tilted and moved. That is, while the tilting control is being performed, the gap, which serves as the tracking margin for performing the tracking control, may be reduced as the lens holder tilts about the tilting axis. To the contrary, while the tracking control is being performed, the gap, which serves as the tilting margin for performing the tilting control, may be reduced as the lens holder moves in the radial direction of the optical medium. Such a reduction of the tracking margin and/or the tilting margin means that a margin for performing the servo control of the optical pick-up actuator is reduced, which contributes to a deterioration in a stability of the servo control. In particular, if an excessively deformed optical medium is used, a situation may arise in which the tilting control cannot be successfully performed because the tilting margin has been reduced, which may cause the optical pick-up actuator to collide with the optical medium.